I just bought Logic a few days ago. I've noticed this problem with GarageBand before, but only thought it was a bug.
I have two instruments looping at the beginning (More Modulation Piano) and (Inst 3). At bar 9 I add in more drums, two different basses, a crash, and a MIDI crash that I turned into an audio track. It is a bar 9 that the two looping instruments get really quiet.
I am positive that I have no automation set.
I looked at the event list for the (Inst 3) region to see if there were any events that were unintended but did not see any.
I've attached a picture of the project. I will gladly take any advice. : ) Thank you!